Oghams

The alphabet in use among the ancient Irish and some other
Celtic nations prior to the ninth century.

“The oghams seem to have been merely tree-runes. The Irish regarded
the oghams as a forest, the individual characters being trees (feada),
while each cross-stroke is called a twig (fleasg).” —IsaacTaylor: The Alphabet, vol. ii. chap. viii. p. 226.